On the one hand, you generally want to be able to add whatever information standard you want - so a standard character sheet seems to make sense, but on the other hand you want quick access to basic relevant information like where they might be found, physical appearance, roleplaying notes, motivations, who their enemies and allies are, and whatever else is important to have in mind when the PCs encounter them.
I think having giving NPCs their own space for DM management, with each NPC having a toggle as to whether players can view/edit them would be very helpful.
